1. Field of the Invention
The present invention relates to a gas sensor control device used for controlling a gas sensor. The gas sensor includes a gas sensor element configured to detect the concentration of a specific gas component in an exhaust or combustion gas of a combustor, an internal-combustion engine, etc.
2. Description of the Related Art
An air-fuel ratio sensor or an oxygen sensor for detecting concentration of oxygen in exhaust gas is known as a gas sensor for controlling combustion or improving fuel efficiency of an internal-combustion engine of a vehicle. In addition, in response to the need for reducing the amount of nitrogen oxide (NOx) in exhaust gas so as to meet strict exhaust gas regulations of a vehicle, an NOx sensor capable of directly measuring concentration of NOx has been developed.
This kind of gas sensor includes a gas sensor element including one or more cells each including an oxygen ion conductive solid electrolyte such as zirconia and a pair of electrodes formed on the solid electrolyte. The concentration of the specific gas is detected based on an output of the gas sensor element.
For example, in an NOx sensor, oxygen in a first measurement chamber communicating with a measurement gas space is pumped out by a first pump cell; an oxygen concentration detecting cell measures oxygen concentration in the first measurement chamber; and the first pumping cell is controlled such that the oxygen concentration in the first measurement chamber maintains a predetermined oxygen concentration. Further, the gas to be measured having a controlled oxygen concentration flows from the first measurement chamber into a second measurement chamber, and a constant voltage is applied to a second pumping cell so as to separate NOx in the gas to be measured into N2 and O2. At this time, a second pump current flowing through a pair of electrodes of the second pumping cell is measured to detect the concentration of NOx in the gas to be measured.
In such an NOx sensor, it is necessary to provide a voltage control circuit for controlling the voltage developed across the electrodes of the oxygen concentration detecting cell or the second pumping cell to maintain the same at a constant voltage. For this reason, a control circuit unit (gas sensor control device) has been developed (JP-A-11-304758).
Further, since the output voltage developed across the cells changes with a change in sensor temperature, the gas detection result changes as well. To deal with this circumstance, JP-A-10-288595 (paragraphs [0073] to [0075]) describes a gas sensor control device that adds a sensor temperature correction amount to the second pump current indicating the NOx concentration, and then outputs a corrected NOx signal.
Incidentally, the gas sensor control device for controlling a gas sensor such as the NOx sensor includes a circuit board connected to the gas sensor via a signal line. On the circuit board, many electronic components (an OP Amp (operational amplifier), a resistor, etc.) constituting the voltage control circuit are mounted. In the gas sensor control device, the voltage control circuit (electronic component and the like) mounted on the circuit board is adjusted so that the voltage developed across the electrodes of the cell is maintained at a constant value before shipping the gas sensor control device. However, such adjustment is carried out only at regular temperatures. For this reason, when a temperature variation occurs at the circuit board in practical use, the control voltage may change (drift). This can happen when a characteristic of the electronic component changes with a change in temperature. In this case, it may not be possible to obtain satisfactory precision in gas concentration detection.
Although JP-A-10-288595 describes a countermeasure against gas detection result variations depending on sensor temperature, the temperature variation of the circuit board is not considered. For this reason, there is a need to solve the problem of control voltage variation due to temperature variation in order to improve the precision of gas concentration detection.